Notes: Blocking sysread() input. Same can be done with $filter->put()
and syswrite(). Be sure to remember get() and put() deal with list
references, not lists. The filters' documentation should cover it.

This is more useful for some of the odder filters, like HTTPD and
Reference, but it's also good if you want to roll your own select() or
IO::Select loop. They also work with other event systems, for what
that's worth.